NAME

       __local - Address Space Qualifier.

       __local local

DESCRIPTION

       OpenCL implements the following disjoint address spaces: __global, __local, __constant,
       and __private The address space qualifier may be used in variable declarations to specify
       the region of memory that is used to allocate the object. The C syntax for type qualifiers
       is extended in OpenCL to include an address space name as a valid type qualifier. If the
       type of an object is qualified by an address space name, the object is allocated in the
       specified address name; otherwise, the object is allocated in the generic address space.

       The address space names without the __prefix i.e.  global, local, constant and private may
       be substituted for the corresponding address space names with the __prefix.

       The generic address space name for arguments to a function in a program, or local
       variables of a function is __private. All function arguments shall be in the __private
       address space.

       functionQualifiers(3clc) function arguments declared to be a pointer of a type can point
       to one of the following address spaces only: __global, __local or __constant. A pointer to
       address space A can only be assigned to a pointer to the same address space A. Casting a
       pointer to address space A to a pointer to address space B is illegal.

       Function arguments of type otherDataTypes(3clc), otherDataTypes(3clc),
       otherDataTypes(3clc), otherDataTypes(3clc), otherDataTypes(3clc), and otherDataTypes(3clc)
       refer to image memory objects allocated in the __global address space.

NOTES

       The __local or local address space name is used to describe variables that need to be
       allocated in local memory and are shared by all work-items of a work-group. Pointers to
       the __local address space are allowed as arguments to functions (including __kernel
       functions). Variables allocated in the __local address space can also be defined inside a
       __kernel function scope.

   Example
       There is no generic address space name for program scope variables. All program scope
       variables must be declared in the __constant address space. For example:

       // declares a pointer p in the __private address
       space that // points to an int object in address
       space __global __global int *p;

       // declares an array of 4 floats in the
       __private address space.  float x[4];

       There is no address space for function return values. Using an address space qualifier in
       a function return type declaration will generate a compilation error, unless the return
       type is declared as a pointer type and the qualifier is used on the points-to address
       space.

       For example:

       __private int f() { ... } // should generate an
       error __local int *f() { ... } // allowed
       __local int * __private f() { ... }; // should
       generate an error.

       // Examples of variables allocated in the
       __local address space // inside a __kernel
       function

       __kernel void my_func(...)  { __local float
       a;       // A single float allocated // in local
       address space

       __local float      b[10];   // An array of 10
       floats // allocated in local address space.  if
       (...)  { // example of variable in __local
       address space but not // declared at __kernel
       function scope.  __local float c;       // not
       allowed.  } }

       Variables allocated in the __local address space inside a __kernel function cannot be
       initialized.

           __kernel void my_func(...)
           {
           local float a = 1;    // not allowed __local float b; b = 1;                // allowed
           }

       NOTE: Variables allocated in the __local address space inside a __kernel function are
       allocated for each work-group executing the kernel.

SPECIFICATION

       OpenCL Specification[1]

SEE ALSO

       global(3clc), constant(3clc), private(3clc)

AUTHORS

       The Khronos Group

COPYRIGHT

       Copyright © 2007-2011y The Khronos Group Inc.
       Permission is hereby granted, free of charge, to any person obtaining a copy of this
       software and/or associated documentation files (the "Materials"), to deal in the Materials
       without restriction, including without limitation the rights to use, copy, modify, merge,
       publish, distribute, sublicense, and/or sell copies of the Materials, and to permit
       persons to whom the Materials are furnished to do so, subject to the condition that this
       copyright notice and permission notice shall be included in all copies or substantial
       portions of the Materials.

NOTES

        1. OpenCL Specification
           page 222, section 6.5 - Address Space Qualifiers